It is common practice to inspect work pieces subsequent to production on a coordinate positioning apparatus, such as a CMM, in order to check for correctness of predefined object parameters, like dimensions and shape of the object.
In a conventional 3-D coordinate measurement machine, a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in space of a measuring volume of the CMM, and the object is measurable with a measurement sensor (probe) carried by the probe head.
In a simple form of the machine, a suitable transducer mounted parallel to each axis is able to determine the position of the probe head relative to a base of the machine and, therefore, to determine the coordinates of a measurement point on the object being approached by the sensor. For providing movability of the probe head, a typical coordinate measuring machine may comprise a frame structure on which the probe head is arranged and driving means for moving frame components of the frame structure relative to each other.
Usually a CMM has a tactile probe or an optical probe for gauging the surface of a target object. The optical or tactile probe is movably fixed at an articulated arm, as it is shown for a tactile probe i.e. in EP 2283311 A1, or at a portal, as it is shown for an optical probe i.e. in WO 2008/135530 A1, so that it can be moved over the surface of the target object.
For measuring surface variations, both measurement principles based on use of tactile sensors and of optical sensors are known.
In general, for providing a coordinate measuring machine with high measurement precision, its frame structure is therefore usually designed to have a high static stiffness. In order to achieve a stiff and rigid machine design, the frame structure or at least parts of it, is often made of stone, such as granite. Besides all the positive effects like high thermal stability and good damping properties, the granite or other stiff materials also make the machine and the movable frame elements quite heavy. The high weight on the other side also requires high forces for a decent acceleration.
Adequate calibrations can make such an approach working quite well, but all non-repetitive imperfections of the mechanics fully impact the resulting accuracy. So, for example, if there is some play between a first axis and a second one, none of the linear scales or angle encoders applied for determining the position of the sensor would enable to recognise such a play.
As a further important aspect, weight reduction is a main topic relating to the designs of novel coordinate measuring machines. If the machine components are built comprising less weight (associated with less stiffness), faster positioning of respective components can be achieved, simultaneously with causing fewer force affecting the coordinate measuring machine. On the other hand, the influence of machine vibrations and torsions caused by reduced stiffness and (faster) movement of the machine components increases along with weight reduction of these parts. Thus, uncertainties of derived measurement values and errors occurring from such deformations and vibrations increase accordingly.
Therefore, especially with regard to weight reduction but also for conventional machines, an accurate error handling is an important aspect.
In industry, measurement times are considered as unproductive times as no salable components are produced during this period. The measurement task therefore has to be done as quickly as possible. Thus, high measurement speed and short preparation times, including a quick fixation of the object to be gauged and a short calibration time for the CMM, is of high commercial importance. In this context, it is understandable that not only the maximum. moving speed of the measurement probe relative to the target object is of interest, which mainly important for large components and long distances to be traveled by the measurement probe, but also a maximum acceleration and deceleration, which important for small work pieces as the latter allows a very fast positioning of the measurement probe on the interesting positions relative to the target object.
Therefore, several measures have been taken during the past in order to increase the measurement speed. E.g., measuring with an optical probe instead of a tactile probe in general can increase the measurement rate and avoid abrasion effects at the surface of the target object.
Another option to increase measurement rate is the use of a camera as a measurement probe and using this camera in an “on the fly”-mode, as it is described in WO 2008/135530 A1. During the “on the fly”-measurement mode, the camera is moved continuously over the target object and takes pictures only at the interesting positions without stopping there. The position data for each image is delivered from position encoders and stored together with the according image. A flash light illumination of the interesting positions ensures a sharp picture in spite of the moving speed of the camera. As the camera is not stopped at the interesting positions, less deceleration and acceleration actions have to be carried out which decreases measuring time and avoids triggering vibrations in the structure. However, in order to know the interesting positions and in order to move the camera along optimized trajectories including all interesting positions, calibration of the CMM, usually on the basis of a reference object, is necessary and a subsequent programming of the trajectories including the defined interesting positions, where pictures should be undertaken.
However, using a normal, cost-efficient camera with a standard sized optics shows a small field of view, when used with a magnification adequate to reach the necessary accuracy. As the field of view is small, it is necessary to take a lot of images, which means a lot of movements of the camera in order to see all features of interest. Thus, the throughput of a commercial CMM with an articulated arm or a portal structure—independent of using it with or without an “on the fly”-mode—is still non-satisfying, caused by its low speed and low acceleration. In order to counter this unsatisfying situation, nowadays many CMMs are offered with cameras provided with objectives of larger diameter, showing a larger field of view for the same magnification. As a result, no movements are necessary to measure small target objects and only a few movements and a few images are necessary for encompassing a large target object. However, those CMMs are expensive, as the price for cameras with an accordingly large objective is high.
The latest development tries to increase the measurement rate by using a so called “Delta robot” instead of a portal machine or an articulated arm for moving a tactile measurement probe (see the brochure “Equator 300 Mess-Systeme” of Renishaw, published in July 2011).
A Delta robot is a kind of parallel robot. It comprises a stationary base fixed at a stationary frame, which is mounted above a workspace, and three middle jointed arms extending from the base. The arms, often called “kinematic chains”, are connected with their first end to the base by means of universal joints and connected with their second end to an end-effector often built in form of a triangular or circular platform.
The arms are typically made of lightweight composite material and are driven by actuators located in the base. Driven by the actuators, the end-effector is movable within a motion zone. The motion zone is the 3-dimensional space the end-effector is maximally movable in. The boundaries of the maximum movement—and accordingly of the motion zone—are defined by the construction of the kinematic chains and the resulting physical limits of their common motion as being linked by the end-effector. Actuation can be done with linear or rotational actuators. As the arms are made of a light composite material, the moving parts of the Delta robot have a small inertia. This allows for very high accelerations and very fast movement, which outclasses by far speed of acceleration and movement realizable by a classical portal machine or an articulated arm.
For the implementation of a Delta robot as a part of a coordinate measuring machine, the degree of freedom of the Delta robot had been extended up to 6, allowing for the end-effector lateral movements in Cartesian directions X, Y, Z, and rotational movements around those axes resulting in yawing, rolling, pitching. Because of their high acceleration/deceleration actions and their high movement speed, Delta robots and machines based on a Delta robot are popular for picking and packaging in factories of the packaging, medical and pharmaceutical industry; some of them executing up to 300 picks per minute.
But in spite of its applicability in various technical fields, Delta robots have widespread been further regarded as not suitable for measurement requirements of coordinate measuring machines. This is because of their sensitivity to temperature fluctuation and strong vibration during fast movement and fast acceleration/deceleration actions, caused by their lightweight construction. As a result, the exact position of the end-effector cannot be determined precisely enough, and adequate focusing with optical means, e.g. an optical probe of a CMM or a camera as arranged conventionally, is not possible.
The usage of a Delta robot for moving a tactile probe of a CMM, as it is proposed in the brochure “Equator 300 Mess-Systeme” mentioned above, is not in contradiction to the statement above, but attempts to avoid these problems associated with Delta robots by using a tactile measurement probe. However, as the tactile probe has to contact the surface of the target object, the tactile probe dictates the measurement speed, which is much slower than what the Delta robot would allow. Thus, the measurement rate is limited by the tactile probe anyway and determination of its position will thus be able. Consequently, the possibilities with respect to acceleration and motion speed, enabled by the Delta structure, are not fully exploited. Furthermore, the CMM disclosed in the brochure referred to above is again only able to determine a target object with respect to a reference object, which reference object has to be measured before measuring the target object. Therefore, it has to be calibrated before the measurement. As a consequence, no decrease of measurement time is possible during preparation of the measurement, and only special trained persons will be able to handle this CMM.
Common to the types of coordinate measuring machines, adequate calibrations may work to a satisfactory extent for taking into account reproducible errors, but all non-repetitive imperfections of the mechanics fully impact a resulting measuring accuracy. So, for example if there is some play between a first axis and a second axis, none of the linear scales or angle encoders applied for determining the position of the CMM probe would have a chance to recognize such a play. Other kinds of imperfections, that are often not compensated for or require at least large efforts for a compensation, include, for example, backlash which has a big impact on the results, and, most of the time, low temperature drifts or any other kind of drifts, which are not well managed.
As far as all those negative effects are known, the usual response is to choose a top quality and expensive mechanics to minimize backlashes or plays. Another consequence is that very complex temperature compensation algorithms are applied, what not is a big issue, but always requires significant data collection phases. Most of the time, the measuring machines individually have to be calibrated at different temperatures, what is long and expensive. Despite all this, allowed temperature drifts still are limited, not really allowing for application in rough plant environments. Finally, in order to compensate for low drifts or effects of collisions, complete recalibrations have to be performed often, usually at least once a year.